Since the discovery of HTSCs, for example YBa.sub.2 Cu.sub.3 O.sub.7-x commonly known as Y-123, there has been a search for appropriate materials on which to grow HTSC thin film devices. Due to the chemical composition, crystal structure and anisotropic superconducting properties of HTSCs, the material used for thin film growth must be chemically compatible, match the HTSCs in temperature expansion coefficient and lattice parameter, exhibit low defect density and be mechanically strong. Epitaxial HTSC films deposited on such materials possess the best superconducting properties. For most device applications, the HTSC is deposited with the c-axis normal to the substrate surface since these fully oxygenated films possess the highest critical temperature (T.sub.c), smallest transition width (.DELTA.T.sub.c), largest critical current density (J.sub.c) and lowest surface resistance (R.sub.S). For microwave device applications requiring fast propagation and/or larger features, the material on which HTSC films are grown must be low loss, have a low dielectric constant and preferably be isotropic with respect to the microwave radiation.
Because garnets have reasonably low dielectric constants of .epsilon. from 9-13, reasonably low dielectric loss of 10.sup.-3, temperature expansion coefficients comparable to those of Y123, and are low cost substrates, they have been suggested as potential substrate materials for the fabrication of HTSC microwave devices. Several groups have attempted growing microwave quality HTSC films on gadolinium gallium garnet (GGG) but none have been successful due to improper lattice matching between GGG and the HTSC.